The invention relates to a nonaqueous coating which contains a polyacrylate resin and a urea group-containing thixotropic agent, and to a process for producing a two-coat finish on a substrate surface.
Processes are known for producing a two-coat finish on a substrate surface, in which
(1) a pigmented basecoat is applied to the substrate surface
(2) the basecoat film applied in step (1) is dried at temperatures from room temperature to 80xc2x0 C.
(3) a transparent topcoat is applied to the basecoat film dried in step (2), and subsequently
(4) basecoat and topcoat are baked together.
Nonaqueous coatings containing a polyacrylate resin and a urea group-containing thixotropic agent are known and are described, for example, in European Patent Application EP-A-192 304, in German Offenlegungsschrift DE 23 59 929, in German Auslegeschriften DE 23 59 923 and DE 18 05 693 and in German Patent DE 27 51 761. The use of thixotropic agents in nonaqueous coatings is intended, inter alia, to enable the application of relatively thick coats without the formation of interfering xe2x80x9crunsxe2x80x9d. It is disadvantageous that nonaqueous coatings containing a polyacrylate resin and a urea group-containing thixotropic agent give rise, especially at high solids contents, to coated surfaces which are unsatisfactory with regard to their visual appearance, especially with regard to evenness and gloss.
The invention accordingly has the object of providing nonaqueous coatings containing a polyacrylate resin and a urea group-containing thixotropic agent which give coating films having surface properties which are improved with respect to the prior art.
This object has surprisingly been achieved by using in the nonaqueous coatings a polyacrylate resin which can be prepared by polymerizing
(a) from 16 to 51, preferably from 16 to 28, % by weight of a hydroxyl group-containing ester of acrylic acid or methacrylic acid or of a mixture of such monomers
(b) from 32 to 84, preferably from 32 to 63, % by weight of an aliphatic or cycloaliphatic ester of acrylic acid or methacrylic acid which is different from (a) and has at least 4 carbon atoms in the alcohol radical or of a mixture of such monomers,
(c) from 0 to 2, preferably from 0 to 1, % by weight of an ethylenically unsaturated carboxylic acid or of a mixture of ethylenically unsaturated carboxylic acids, and
(d) from 0 to 30, preferably from 0 to 20, % by weight of an ethylenically unsaturated monomer which is different from (a), (b) and (c) or of a mixture of such monomers
to give a polyacrylate resin having an acid number of from 0 to 15, preferably from 0 to 8, a hydroxyl number of from 80 to 140, preferably from 80 to 120, and a number-average molecular weight of from 1,500 to 10,000, preferably from 2,000 to 5,000, the sum of the proportions by weight of components (a), (b), (c) and (d) always being 100% by weight.
The polyacrylate resins employed in accordance with the invention can be prepared by polymerization processes which are generally well known. Polymerization processes for the preparation of polyacrylate resins are generally known and described in many references (cf. e.g.: Houben-Weyl, Methoden der organischen Chemie, [Methods of Organic Chemistry], 4th Edition, Volume 14/1, page 24 to 255 (1961)).
The polyacrylate resins employed in accordance with the invention are preferably prepared using the solution polymerization process. Conventionally, in this process an organic solvent or solvent mixture is initially introduced and heated to the boil. To this organic solvent or solvent mixture are then added, continuously, the monomer mixture to be polymerised and one or more polymerisation initiators. Polymerisation is carried out at temperatures of between 100 and 160xc2x0 C., preferably between 130 and 150xc2x0 C. As polymerisation initiators it is preferred to employ free-radical initiators. The nature and quantity of initiator are usually chosen so that the supply of free radicals present during the feed phase at the polymerisation temperature is substantially constant.
Examples of initiators which can be employed are: dialkyl peroxides such as di-tert-butyl peroxide or dicumyl peroxide; hydroperoxides such as cumene hydroperoxide or tert-butyl hydroperoxide; and peresters such as tert-butyl perbenzoate, tert-butyl perpivalate, tert-butyl per-3,5,5-trimethylhexanoate or tert-butyl per-2-ethylhexanoate.
The polymerization conditions (reaction temperature, feed time of the monomer mixture, nature and quantity of the organic solvents and polymerisation initiators, possible additional use of molecular weight regulators, for example mercaptans, thioglycolic acid esters and hydrogen chlorides) are selected so that the polyacrylate resins employed in accordance with the invention have a number-average molecular weight of from 1,500 to 10,000, preferably from 2,000 to 5,000 (determined by gel permeation chromatography using polystyrene as calibration substance).
The acid number of the polyacrylate resins employed in accordance with the invention can be adjusted by the person skilled in the art by using appropriate quantities of component (c). Analogous comments apply to the adjustment of the hydroxyl number. It can be controlled via the quantity of component (a) employed.
It is possible in principle to employ as component (a) any hydroxyl group-containing ester of acrylic acid or methacrylic acid, or a mixture of such monomers. Examples are: hydroxyalkyl esters of acrylic acid, for example hydroxyethyl acrylate, hydroxypropyl acrylate and hydroxybutyl acrylate, especially 4-hydroxybutyl acrylate; hydroxyalkyl esters of methacrylic acid, for example hydroxyethyl methacrylate, hydroxypropyl methacrylate and hydroxybutyl methacrylate, especially 4-hydroxybutyl methacrylate; and reaction products of cyclic esters, for example xcex5-caprolactone, and hydroxyalkyl esters of acrylic acid and/or methacrylic acid.
The composition of component (a) is preferably selected such that the polyacrylate resin resulting from homopolymerization of component (a) has a glass transition temperature of from xe2x88x9250 to +70, preferably from xe2x88x9230 to +50xc2x0 C. The glass transition temperature can be calculated approximately by the person skilled in the art using the formula       1          T      G        =            ∑              n        =        1                    n        =        x              ⁢          xe2x80x83        ⁢                  W        n                    T        Gn            
TG=glass transition temperature of the polymer
x=number of different copolymerised monomers,
Wn=proportion by weight of the nth monomer
TGn=glass transition temperature of the homopolymer of the nth monomer.
It is possible in principle to employ as component (b) any aliphatic or cycloaliphatic ester of acrylic acid or methacrylic acid which is different from (a) and has at least 4 carbon atoms in the alcohol radical, or a mixture of such monomers. Examples are: aliphatic esters of acrylic and methacrylic acid having from 4 to 20 carbon atoms in the alcohol radical, for example n-butyl, iso-butyl, tert-butyl, 2-ethylhexyl, stearyl and lauryl acrylate and methacrylate and cycloaliphatic esters of acrylic and methacrylic acid, for example cyclohexyl acrylate and cyclohexyl methacrylate. The composition of component (b) is preferably selected such that the polyacrylate resin resulting from homopolymerization of component (b) has a glass transition temperature of from 10 to 100, preferably from 20 to 60xc2x0 C.
It is possible in principle to employ as component (c) any ethylenically unsaturated carboxylic acid or a mixture of ethylenically unsaturated carboxylic acids. As component (c) it is preferred to employ acrylic acid and/or methacrylic acid.
It is possible in principle to employ as component (d) any ethylenically unsaturated monomer which is different from (a), (b) and (c), or a mixture of such monomers. Examples of monomers which can be employed as components (d) are: vinyl aromatic hydrocarbons, such as styrene, xcex1-alkyl styrene and vinyl toluene, amides of acrylic acid and methacrylic acid, for example methacrylamide and acrylamide; nitriles of methacrylic acid and acrylic acid; vinyl ethers and vinyl esters. It is preferred to employ as component (d) vinyl aromatic hydrocarbons, especially styrene.
The composition of component (d) is preferably selected such that the resin resulting from homopolymerisation of component (d) has a glass transition temperature of from 70 to 120, preferably from 80 to 100xc2x0 C.
The urea group-containing thixotropic agents contained in the nonaqueous coatings according to the invention are known and are described, for example, in detail in German Offenlegungsschrift DE 23 59 929, in German Auslegeschriften DE 18 05 693 and DE 23 59 923 and in German Patent DE 27 51 761. They are prepared by reacting a compound containing isocyanate groups, or a mixture of compounds containing isocyanate groups, with primary and/or secondary amines and/or water.
The urea group-containing thixotropic agents employed in the nonaqueous coatings according to the invention are preferably prepared by reacting monoamines, or mixtures of monoamines, with polyisocyanates or mixtures of polyisocyanates, the monoamines and the polyisocyanates being reacted with one another in quantities such that the ratio of equivalents between amino groups and isocyanate groups is between 1.2 and 0.4, preferably between 1.0 and 0.8. The monoamines employed are preferably primary monoamines, particularly preferably araliphatic or aliphatic primary monoamines and very particularly preferably aliphatic primary monoamines having at least 6 carbon atoms in the molecule. Examples of monoamines which can be employed are: benzylamine, ethylamine, n-propylamine, isopropylamine, n-butylamine, isobutylamine, tert-butylamine, pentylamine, n-hexylamine, n-octylamine, iso-nonanylamine, iso-tridecylamine, n-decylamine and stearylamine.
The polyisocyanates which it is possible to employ in principle are all organic compounds containing at least two isocyanate groups per molecule. It is also possible to employ isocyanate group-containing reaction products of, for example, polyols and polyamines and polyisocyanates. It is preferred to employ diisocyanates, very particularly preferably aliphatic diisocyanates and, in particular, hexamethylene diisocyanate. Example [sic] of polyisocyanates which can be employed are: tetramethylene 1,4-diisocyanate, hexamethylene 1,6-diisocyanate, xcfx89,xcfx89xe2x80x2-dipropyl ether diisocyanate, cyclohexyl 1,4-diisocyanate, dicyclohexylmethane 4,4xe2x80x2-diisocyanate, 1,5-dimethyl(2,4-xcfx89-diisocyanato-methyl)benzene, 1,5-dimethyl(2,4-xcfx89-diisocyanato-ethyl)benzene, 1,3,5-trimethyl(2,4-xcfx89-diisocyanato-methyl)benzene, 1,3,5-triethyl(2,4-xcfx89-diisocyanato-methyl)benzene, the trimer of hexamethylene 1,6-diisocyanate, isophorone diisocyanate, dicyclohexyldimethylmethane 4,4xe2x80x2-diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, diphenylmethane 4,4xe2x80x2-diisocyanate.
The urea group-containing thixotropic agent is preferably prepared in the presence of the polyacrylate resin employed in accordance with the invention. The procedure here is usually such that the amine component is added to a solution of the acrylate resin in an organic solvent or in a mixture of organic solvents and then the polyisocyanate is added as quickly as possible and with very vigorous stirring. The resulting mixture of urea group-containing thixotropic agent and polyacrylate resin can then be employed in the nonaqueous coatings according to the invention.
The nonaqueous coatings according to the invention contain from 30 up to 70, preferably from 40 to 60, % by weight of organic solvents, for example: aliphatic, aromatic and cycloaliphatic hydrocarbons, alkyl esters of acetic acid or propionic acid, alkanols, ketones and glycol ethers and glycol ether esters.
The nonaqueous coatings according to the invention may also contain, in addition to the organic solvents, the polyacrylate resin and the urea group-containing thixotropic agent, crosslinking agents, further binders which are compatible with the polyacrylate resin employed in accordance with the invention, pigments, fillers, light stabilizers and other additives typical for coatings.
It is preferred for the nonaqueous coatings to contain from 25 to 100, preferably from 30 to 70, % by weight, based on the solids content of polyacrylate resin, of a crosslinking agent or of a mixture of crosslinking agents. Examples of crosslinking agents which can be employed are amino resins, especially etherified melamine formaldehyde condensates and blocked and unblocked polyisocyanates, and mixtures of these crosslinking agents. The crosslinking agent is added in a quantity such that the ratio of equivalents between the reactive groups of the binder and the reactive groups of the crosslinking agent is between 1.5 to 0.5 and 0.5 to 1.5, preferably between 1.2 to 0.8 and 0.8 to 1.2.
The nonaqueous coatings according to the invention contain the urea group-containing thixotropic agent in a quantity of from 0.1 to 30.0, preferably from 0.5 to 10 and particularly preferably from 0.1 to 5.0% by weight, based on the total solids content of the nonaqueous coatings according to the invention.
The nonaqueous coatings according to the invention can be applied using conventional application methods, especially by spraying, to any desired substrate, especially to metals, wood, plastic etc.
The coated surfaces which can be produced using the nonaqueous coatings according to the invention have surface properties which are sufficiently outstanding to enable the coatings also to be employed for coating car bodies, especially as transparent coatings in the production of two-coat finishes of the basecoat/clearcoat type. Two-coat finishes of the basecoat/clearcoat type are prepared by
(a) applying a pigmented basecoat to the substrate surface
(2) drying the basecoat film applied in step (1) at temperatures from room temperature to 80xc2x0 C.
(3) applying a transparent topcoat to the basecoat film dried in step (2), and subsequently
(4) baking the basecoat and topcoat together.